This invention relates to new binder combinations comprising OH-functional graft copolymer resins and multi-functional crosslinking resins, preferably polyisocyanate resins.
The binders according to this invention are suitable, for example, for use in the motor vehicle sector, especially for coating parts which are exposed to severe impact and bending stresses and severe weathering conditions.
It is known that the properties of plastics may be improved to such an extent by means of lacquering that they become suitable for use in special applications. If, for example, parts made of plastics for the motor vehicle industry are to be used in situations where they are exceptionally exposed to weathering, impacts and environmental influences as, for example, in the region of the lower side parts (sills) and the front and rear parts, these parts must be covered with coatings which have high flexibility and surface hardness.
Plastics surfaces which are particularly exposed to road dirt (sills) must be protected by coatings which not only have the necessary mechanical resistance (resistance to stone chipping) but also a high resistance to salt, fuels, and tar and oil stains. All the coatings must have good optical characteristics as films so that they not only have a high degree of gloss but also retain these optical properties over a period of many years in spite of the influence of weathering.
It is known from DE-OS No. 2 364 736 to coat rubber products with an acrylic lacquer based on styrene, alkyl (meth) acrylic acid esters and hydroxy alkyls (meth) acrylic acid esters in combination with alkylated melamine-formaldehyde resins or polyisocyanates. The highly flexible coatings thus obtained are not always entirely satisfactory, for example in their mechanical properties, in the adherence of the coating to the plastic surface, etc.
Before the rubber surface is coated, it must be surface activated and then primed with two coats of acrylic resin as the coatings only manifest their special properties on pre-treated surfaces of rubber such as an ethylene-propylene copolymer rubber, a styrene-butadiene copolymer rubber, a polybutadiene rubber, a polyisoprene rubber or a butyl rubber.
Films based on polyacrylates which can be coated with a reactive lacquer system based on polyols/polyisocyanate without requiring a preliminary treatment and without problems of adherence are disclosed in DE-OS No. 2 756 708. The elimination of problems of adherence is achieved by adding to the copolymers from 1 to 15% by weight of monomers with functional end groups which are capable of forming a bond with the isocyanate-based reactive lacquer system. Although this process has the advantage of eliminating the special preliminary treatment of the substrate surface, it has the disadvantage that it is only applicable to the special case of films on (meth) acrylate copolymers and cannot be used for coating substrate surfaces based on other polymers.
It has now been found that binder combinations of OH-functional graft copolymer resins and polyisocyanates form firmly adhering films on surfaces which have not been pre-treated, preferably metal or plastics surfaces, after they have been cured. Their excellent overall properties such as elasticity, hardness, chemical resistance and weathering resistance enables them to be used e.g. in the construction of motor vehicles by the composite construction method or for the construction of windows, household implements, etc.